The dimension of an internal reflection (ATR) element (IRE) is given by its length (l), width (w), and thickness (t). The length of the IRE, measured from the center of the entrance aperture to the center of the exit aperture, determines the number of reflections, N.

N = ( l / t ) tan θ

If l is kept constant, plates of a different thickness can be exchanged without the necessity of realigning the accessory.